The current generation of Li-ion batteries use liquid electrolyte; although, from a safety perspective a solid state electrolyte would be preferred. The current generation of Li-ion batteries consist of a positive electrode and a negative electrode separated by a porous separator and liquid electrolyte used as the ionic conductive matrix. In these Li-ion batteries, the liquid electrolyte represents a safety hazard because it is flammable and reacts with the other cell components. Shorting of the negative electrode to the positive electrode can cause a fire. Shorts may be caused by one or more of: (a) conductive asperities or particles in the cell which are introduced during manufacturing; (b) dendrites that grow from one electrode to the other during operation of the cell; and (c) shrinking of the separator due to overheating. To prevent shorts, cells are currently designed with thick, strong separators that may incorporate advanced structures—for example, separators impregnated or coated with ceramic nano-particles. Also, reactions between the electrolytes and the other active materials in the cell can result in nominally identical cells having different rates of capacity aging. This makes series stacking difficult as the imbalance reduces the available capacity of the series stack and can result in safety issues—for example, over charging of some cells in the battery due to stacking of cells with different capacities may cause premature failure or thermal runaway of overcharged cells. These potential problems are addressed in today's batteries as follows: (1) by incorporating safety elements in the cells—pressure release vents and switches, and PTC (positive temperature coefficient) current limiters; (2) monitoring the battery pack by the battery pack electronics—e.g. monitoring temperature, voltage of each cell or parallel set, total stack voltage and total pack current; and (3) by using protective battery enclosures and, sometimes, active cooling. All of these measures add expense and reduce the energy density at the cell and pack level.
There is a need for Li-ion batteries with nonflammable solid state electrolytes that can avoid the aforementioned problems associated with today's liquid electrolyte cells.